Superabsorbers are water-insoluble, crosslinked polymers which are able to absorb large amounts of aqueous fluids, especially body fluids, more especially urine or blood, with swelling and the formation of hydrogels, and to retain such fluids under a certain pressure. By virtue of those characteristic properties, such polymers are chiefly used for incorporation into sanitary articles, such as, for example, baby's nappies/diapers, incontinence products or sanitary towels.
The preparation of superabsorbers is generally carried out by free-radical polymerisation of acid-group-carrying monomers in the presence of crosslinkers, it being possible for polymers having different absorber properties to be prepared by the choice of the monomer composition, the crosslinkers and the polymerisation conditions and of the processing conditions for the hydrogel obtained after the polymerisation (for details see, for example, Modern Superabsorbent Polymer Technology, F L Buchholz, G T Graham, Wiley-VCH, 1998).
The acid-group-carrying monomers can be polymerized in the presence of the crosslinkers in a batch process or in a continuous process. An example for a continuous process for preparing superabsorbers is disclosed in U.S. Pat. No. 4,857,610. According to this process, an aqueous monomer solution containing acrylic acid and crosslinkers is continuously charged in a layer at least one centimeter thick on a moving endless conveyor belt and polymerized. The gel that is thus obtained is continuously discharged from the conveyor belt and is subsequently comminuted and dried to a water content of preferably less than 10 wt.-%.
Drying of the polymer gel particles is usually performed on a continuous belt dryer. US 2008/214749 A1 discloses a process for producing a water-absorbing polymers having a low Drying Quality Index by polymerizing a monomer solution, comprising at least one ethylenically unsaturated acid-functional monomer and at least one crosslinker, and drying the resulting hydrogel by means of a heated gas stream in a continuous belt dryer. According to this process a gas stream is flowed against the hydrogel upwardly in an upstream sector of a belt dryer and downwardly in a downstream sector of a belt dryer.
The upwardly directed flow of the hot drying gas onto the belt material in the upstream sector of the belt dryer in the process disclosed in US 2008214749 A1 heats the belt material to temperatures significantly above 180° C. However, the inventors of the present invention have discovered that if the gel particles are charged onto such a heated surface in the upstream sector of the belt dryer, this may lead to a product with insufficient and unreproducible properties. For example the color may change by charging the gel particles onto a belt surface that is to hot. Additionally the product may stick to the conveyor belt if the heat is too high at the location at which the gel is charged onto the belt.